Scientists have found a way to stop cannabis users feeling the key mood changes associated with the drug.

Medication produced from this chemical could help relieve any psychological dependence felt by cannabis users trying to stop taking it, the researchers say.

Cannabis works by releasing chemicals which can bind to "receptor sites" in the brain, in turn leading to the release of mood-altering chemicals within the organ.

These "cannabinoid receptors" are densely packed in the regions of the brain concerned with learning and memory, attention and control of movement.

Dr Marilyn Huestis at the National Institute on Drug Abuse in Baltimore, tested the effects of a chemical developed to block these receptor sites, preventing the cannabinoids from reaching their target.

Volunteers were either given the "blocking" chemical, or a dud placebo, and then smoked one marijuana cigarette.

Some users find that they rely on cannabis to get them through the day, but there is no physical dependency to the drug

Spokesman, Drugscope

Those who had taken the blocking formula showed significantly reduced marijuana effects.

Increasing doses of the chemical appeared to have an increasing effect on the sensation associated with cannabis.

Other physical effects of cannabis were reduced.

Volunteers given the highest dose of the chemical had an increase in heart rate, but only 59% of what might be expected if cannabis alone had been taken.

Dr Alan Leshner, the director of the National Institute on Drug Abuse, said: "This research helps point the way toward possible treatment for those addicted to marijuana and perhaps may be useful in finding effective treatments for other disorders related to the cannabinoid system."

Controversial theories

The long-term effects of cannabis on the body are still a matter of controversy.

While researchers at the NIDA have released studies which they say point to the possibility of physical addiction to the drug, other scientists say that this is impossible.

A spokesman for the UK advice group Drugscope said a degree of psychological addiction was present in some users, but no more than this.

He said: "Some users find that they rely on cannabis to get them through the day, but there is no physical dependency to the drug.

"If this chemical helps release them from this psychological bond, then it might be of some use."

Medications already exist to help wean addicts off drugs such as heroin by making them feel desperately ill should they take the drug while the medicine is in their body.